In color picture tubes, the accuracy with which the electron beams, emitted from an electron gun, strike individual elemental cathodoluminescent screen areas, depends to a great degree upon the accuracy with which the shadow mask apertures are aligned with the elemental screen areas during operation of the tube. During initial tube operation, the shadow mask is heated by the impingement of the electron beams thereon at a much faster rate than is the heavier peripheral frame. Because of the lack of heat input to the mask periphery, the apertured portion of the shadow mask expands at a greater rate than does its periphery. The mask periphery acts as a cold band around the apertured portion and causes the apertured portion to dome. Such doming causes the electron beams, passing through the mask, to misregister with their associated elemental screen areas, thereby producing an undesirable shift in color on the screen.
In most current color picture tube types, the peripheral frame is suspended in a faceplate panel by means of springs that either are directly welded to the frame or are welded to plates which in turn are welded to the frame. In the directly welded version, the springs are made of bimetallic metals, and in the plate version, the plates are of bimetallic material. As the springs or plates become heated by transfer of heat from the mask through the frame, the bimetallic materials expand differently, thereby bending the springs or plates to cause movement of the mask-frame assembly toward the tube screen.
It is common to use either three or four springs to support a mask-frame assembly within a rectangular faceplate panel of a tube. In a three spring support system, one spring is usually located at the upper center of the mask, and the other two springs are located along the sides of the tube between the centers of the sides of the mask and the lower two corners of the mask. In a four-spring support system, springs are usually located at the top and bottom centers of the mask and at the left and right centers of the mask. In both the three- and four-spring support systems, as described above, it is possible for the mask-frame assembly to slightly twist and shift relative to the faceplate during tube operation.
There is a need to develop a tube with a mask-frame assembly and support system that will minimize the mask doming, mask twisting and shifting problems, and that also will be economical to construct. The present invention fulfills such a need.